The present invention relates to a method of steadily forming, on a semiconductor substrate, a silica-containing film having characteristics of a low-dielectric constant as low as 3 or less, a low moisture adsorptivity and high film strength without causing any damage to the metal wiring arranged on the substrate, and also relates to a semiconductor substrate coated with a silica-containing film having the above characteristics.
In semiconductor devices with a multi-level integrated circuit designed at a rule of 0.25 micron or less, the metal wiring impedance attributed to electrostatic induction is increased because of the narrow spacing between metal wiring layers required for the advanced integration of such semiconductor devices. Thus, a delay in response speed and an increase in power consumption become problems to be solved. To cope with this, it is necessary to minimize the dielectric constant of an interconnect insulating film disposed between a semiconductor substrate and metal wiring layer such as an aluminum wiring layer or between metal wiring layers.
The interconnect insulating film disposed for the above purposes is generally formed on a substrate by a vapor phase growth method such as CVD method or by an ordinary coating method which uses a conventional coating liquid.
However, it is considered that the dielectric constant of the silica-containing film produced by the vapor phase growth method such as CVD method is limited to 3.5 (which is obtainable only in the case of the fluorine-doped silica-containing film), and that decreasing it to 3 or less is difficult. On the other hand, the coating film containing polyaryl resin, fluorinated polyimide resin or fluoro-resin, formed on a substrate by the CVD method or the ordinary coating method, exhibits the low-dielectric constant of around 2. However, the coating film containing such a resin has other types of drawbacks such as insufficient adhesion to the substrate or resist used for its fine processing, and poor resistance to chemicals and oxygen plasma.
The conventional coating liquid for forming a silica-containing film, which comprises hydrolyzate of alkoxysilane or halogenated silane and/or a partial hydrolyzate thereof, gives a coating film with the low-dielectric constant of around 2.5. However, there is a problem of the coating film insufficiently adhering to the substrate.
The inventors of the present invention have found, after extensive studies to solve the above problems, that each of the coating liquids described below gives a silica-containing film with the low-dielectric constant of 3 or less, and being excellent not only in adhesion to the substrate, mechanical strength, resistance to chemicals (e.g., alkali) and anti-cracking, but also in process suitability (e.g., resistance to oxygen plasma and etching processability). Based on such findings, they have filed several patent applications as follows:
(1) Invention 1 (Japanese Patent Laid-Open Publication No. 315812/1997)
A coating liquid comprising a reaction product between fine particles of silica, and alkoxysilane and/or halogenated silane or hydrolyzate thereof.
(2) Invention 2 (International Publication No. WO 00/18847)
A coating liquid comprising alkoxysilane and/or halogenated silane or hydrolyzate thereof, and a readily decomposable resin which decomposes or vaporizes at a temperature of 500xc2x0 C. or lower.
(3) Invention 3 (International Publication No. WO 00/12640)
A coating liquid comprising polysiloxane as a reaction product between fine particles of silica, and alkoxysilane and/or halogenated silane or hydrolyzate thereof, and a readily decomposable resin which decomposes or vaporizes at a temperature of 500xc2x0 C. or lower.
Thereafter, the inventors of the present invention have repeatedly conducted tests for forming a low-dielectric silica coating film from the above coating liquids on various semiconductor substrates by the ordinary film forming method, and then they have found that, although a coating film having the above characteristics can be obtained, metal wiring arranged on some types of the substrates may be damaged, and that it is difficult to stably form the coating film having excellent characteristics, that is, a low-dielectric constant of 3 or less, a low moisture adsorptivity and high film strength. Therefore, they have further continued the investigations to solve the above problems, and have found that such problems can be easily solved by forming a low-dielectric, silica-containing film on a semiconductor substrate under specific conditions as described below.
The present invention has been accomplished with a view to solving the above problems of the prior art.
Accordingly, it is an objective of the present invention to provide a method of steadily forming, on a semiconductor substrate, a low-dielectric, silica-containing film having characteristics of a low-dielectric constant of 3 or less, a low moisture adsorptivity and high film strength without causing any damage to the metal wiring arranged on the substrate. It is another objective of the present invention to provide a semiconductor substrate coated with a silica-containing film having the above characteristics.
The method of the present invention for forming a silica-containing film with a low-dielectric constant of 3 or less steadily on a semiconductor substrate comprises steps of:
(a) applying a coating liquid for forming a silica-containing film with a low-dielectric constant onto a semiconductor substrate,
(b) heating the thus coated film at 50 to 350xc2x0 C., and then
(c) curing the thus treated film at 350 to 450xc2x0 C. in an inert-gas atmosphere containing 500 to 15,000 ppm by volume of oxygen.
The coating liquid for forming a silica-containing film with a low-dielectric constant preferably contains hydrolyzate of at least one silicon compound selected from the group consisting of alkoxysilanes and halogenated silanes represented by the following general formula (I) and (II), respectively:
XnSi(OR)4-nxe2x80x83xe2x80x83(I),
XnSiXxe2x80x24-nxe2x80x83xe2x80x83(II),
wherein, X represents a hydrogen atom, fluorine atom, unfluorinated or fluorinated alkyl group of 1 to 8 carbon atoms, aryl group or vinyl group; R represents a hydrogen atom, alkyl group of 1 to 8 carbon atoms, aryl group or vinyl group; Xxe2x80x2 represents a halogen atom; and n is an integer of 0 to 3.
The coating liquid for forming a silica-containing film with a low-dielectric constant also preferably contains a compound of polysiloxane as a reaction product between fine particles of silica and hydrolyzate of at least one silicon compound selected from the group consisting of alkoxysilanes and halogenated silanes represented by the following general formula (I) and (II), respectively:
XnSi(OR)4-nxe2x80x83xe2x80x83(I),
XnSiXxe2x80x24-nxe2x80x83xe2x80x83(II),
wherein, X represents a hydrogen atom, fluorine atom, an unfluorinated or fluorinated alkyl group of 1 to 8 carbon atoms, aryl group or vinyl group; R represents a hydrogen atom, alkyl group of 1 to 8 carbon atoms, aryl group or vinyl group; Xxe2x80x2 represents a halogen atom; and n is an integer of 0 to 3.
The coating liquid for forming a silica-containing film with a low-dielectric constant also preferably contains a readily decomposable resin having a number-average molecular weight of 500 to 50,000 in terms of polystyrene, and decomposing or vaporizing easily when thermally treated at a temperature of 450xc2x0 C. or lower.
The readily decomposable resin is preferably constituted with a composition of an interpenetrated polymer in which the hydrolyzate of the silicon compound or polysiloxane and the readily decomposable resin are homogeneously entangled in each other on the molecular chain level.
The coating liquid for forming a silica-containing film with a low-dielectric constant preferably has the Sixe2x80x94H group at the terminal of any one of the hydrolyzate of the silicon compound, the polysiloxane and/or the interpenetrated polymer.
The thermal treatment step (b) as described above is preferably conducted at 150 to 350xc2x0 C. for 1 to 3 minutes in an air atmosphere, especially by use of a dry air.
The curing step (c) as described above is preferably conducted in an inert-gas atmosphere containing 1,000 to 10,000 ppm by volume of oxygen.
The above curing step (c) is also preferably conducted by placing the semiconductor substrate on a hot plate kept at a temperature of 350 to 450xc2x0 C.
On the other hand, the semiconductor substrate of the present invention is characterized in that it has the silica-containing film with a low-dielectric constant of 3 or less and other excellent characteristics as described herein, which is formed by use of the above specific method.
The method for forming a silica-containing film (i.e., a coating film) with a low-dielectric constant on a semiconductor substrate according to the present invention is described below in detail.
The method of the present invention can use some kinds of conventional coating liquids for forming a silica-containing film with a low-dielectric constant although depending on their properties, but preferably uses at least one of the following coating liquids (Coating Liquid-A, Coating Liquid-B and Coating Liquid-C) to form a silica-containing film with a low-dielectric constant on a semiconductor substrate.
(1) Coating Liquid-A
Coating Liquid-A contains a reaction product obtained by hydrolyzing or partial-hydrolyzing at least one silicon compound mixed with an organic solvent in the presence of a catalyst and water, wherein the silicon compound is selected from the group consisting of alkoxysilanes and halogenated silanes represented by the following general formula (I) and (II), respectively:
XnSi(OR)4-nxe2x80x83xe2x80x83(I),
XnSiXxe2x80x24-nxe2x80x83xe2x80x83(II),
wherein, X represents a hydrogen atom, fluorine atom, fluorinated or unfluorinated alkyl group of 1 to 8 carbon atoms, aryl group or vinyl group; R represents a hydrogen atom, alkyl group of 1 to 8 carbon atoms, aryl group or vinyl group; xxe2x80x2 represents a halogen atom; and n is an integer of 0 to 3.
Examples of the alkoxysilanes represented by the general formula (I) include methyltrimethoxysilane, methyltriethoxysilane, methyltriisopropoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltriisopropoxysilane, octyltrimethoxysilane, octyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, trimethoxysilane, triethoxysilane, triisopropoxysilane, fluorotrimethoxysilane, fluorotriethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, diethyldimethoxysilane, diethyldiethoxysilane, dimethoxysilane, diethoxysilane, difluorodimethoxysilane, difluorodiethoxysilane, trifluoromethyltrimethoxysilane, and trifluoromethyltriethoxysilane.
Examples of the halogenated silanes represented by the general formula (II) include trichlorosilane, tribromosilane, dichlorosilane, fluorotrichlorosilane, fluorobromosilane, methyltrichlorosilane, ethyltrichlorosilane, phenyltrichlorosilane, and vinyltrichlorosilane.
As for the organic solvents, alcohols, ketones, ethers, esters and hydrocarbons, for example, can be used. More concretely, they include alcohols, e.g., methanol, ethanol, propanol and butanol; ketones, e.g., methylethylketone and methylisobutylketone; glycol ethers, e.g., methylcellosolve, ethylcellosolve and propyleneglycol-monopropylether; glycols, e.g., ethylene glycol, propylene glycol and hexylene glycol; esters, e.g., methyl acetate, ethyl acetate, methyl lactate and ethyl lactate; hydrocarbons, e.g., hexane, cyclohexane and octane; and aromatic hydrocarbons, e.g., toluene, xylene and mesitylene.
The catalysts include inorganic acids, e.g., hydrochloric, nitric and sulfuric acid; organic acid, e.g., acetic, oxalic and toluenesulfonic acid; and compounds exhibiting acidity in an aqueous solution, e.g., metallic soap.
Water, which is necessary for the hydrolysis reaction of alkoxysilane or halogenated silane, is added in an amount of 0.1 to 5 mols, preferably 0.1 to 2 mols, per mol of the Sixe2x80x94OR group which constitutes the alkoxysilane or the Sixe2x80x94Xxe2x80x2 group which constitutes the halogenated silane. The catalyst is normally added in an amount of 0.001 to 1 mol per mol of the alkoxysilane or halogenated silane.
The hydrolysis reaction of alkoxysilane is performed normally at a temperature of 80xc2x0 C. or lower, preferably 5 to 60xc2x0 C. under agitation for 10 hours or less, preferably 0.5 to 5 hours. The hydrolysis reaction of halogenated silane is normally performed at a temperature of 50xc2x0 C. or lower, preferably 5 to 20xc2x0 C. under agitation for a period of 20 hours or less, preferably 1 to 10 hours. When the alkoxysilane and halogenated silane are simultaneously hydrolyzed, the above conditions for the halogenated silane are preferable to be selected.
Thus obtained hydrolyzate or partial hydrolyzate desirably has a number-average molecular weight, in terms of polystyrene, of 500 to 10,000, preferably 1,000 to 5,000.
(2) Coating Liquid-B
Coating Liquid-B contains a compound of polysiloxane as a reaction product between fine particles of silica and hydrolyzate of at least one silicon compound selected from the group consisting of alkoxysilanes and halogenated silanes represented by the above formula (I) and (II). The fine particles of silica are obtained by mixing at least one of the alkoxysilanes represented by the formula (I) with an organic solvent, and then hydrolyzing and polycondensing the alkoxysilane in the presence of water and ammonia. The polysiloxane is obtained by mixing the fine particles of silica and at least one of a silicon compound selected from the group consisting of alkoxysilanes and halogenated silanes represented by the above formula (I) and (II) with an organic solvent, and then hydrolyzing them in the presence of water and catalyst.
The detailed methods of preparing the polysiloxane are described in the patent specifications of Invention 1 (Japanese Patent Laid-Open Publication No. 315812/1997) and Invention 3 (WO00/12640).
(3) Coating Liquid-C
Coating Liquid-C contains, in addition to the hydrolyzate or partial hydrolyzate for Coating Liquid-A or the polysiloxane for Coating Liquid-B, a readily decomposable resin.
The readily decomposable resin used in the present invention has a number-average molecular weight of 500 to 50,000 in terms of polystyrene, and decomposes or vaporizes by thermal treatment at a temperature of 450xc2x0 C. or lower.
Examples of the suitable readily decomposable resins include cellulose-resins, polyamide-resins, polyester-resins, acrylic-resins, polyether-resins, polyolefin-resins, polyol-resins and epoxy-resins.
The number-average molecular weight of these readily decomposable resins is in the range of 500 to 50,000, preferably 5,000 to 30,000 in terms of polystyrene.
In Coating Liquid-C, the readily decomposable resin is preferably constituted with a composition of an interpenetrated polymer in which the hydrolyzate of the silicon compound or polysiloxane and the readily decomposable resin are homogeneously entangled in each other on the molecular chain level. This interpenetrated polymer composition also has a number-average molecular weight, in terms of polystyrene, of 500 to 50,000, preferably 1,000 to 30,000.
The detailed methods of preparing the interpenetrated polymer composition are described in the patent specifications of Invention 2 (WO00/18847) and Invention 3 (WO00/12640).
The coating liquids suitably used in the present invention include Coating Liquids A to C as described above. Furthermore, they preferably have the Sixe2x80x94H group at the terminal of the hydrolyzate of the silicon compound, the polysiloxane and/or the interpenetrated polymer.
The Sixe2x80x94H group is oxidized, when the coated film is cured in a specific gas atmosphere in the present invention (e.g., in an atmosphere of nitrogen gas containing some amount of oxygen), by the presence of a small but adequate quantity of oxygen (e.g., 500 to 15,000 ppm by volume), in the following order:
Sixe2x80x94Hxe2x86x92SiOHxe2x86x92SiO2,
where it is likely to form the networks of xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94Oxe2x80x94 in the cured film.
The silica-containing film with a low-dielectric constant of 3 or less, a low moisture adsorptivity and high film strength can be easily formed, when any one of Coating Liquids A to C is used.
The above coating liquid of the present invention preferably contains solid components such as the hydrolyzate of the silicon compound, the polysiloxane, the readily decomposable resin, and/or the interpenetrated polymer at a concentration of 5 to 30% by weight, more preferably 10 to 25% by weight in an organic solvent.
The organic solvents may be selected from the group consisting of alcohols, glycol ethers, ketones, other types of ethers, esters, and hydrocarbons including aromatic hydrocarbons.
The solution containing the solid components, prepared by the above method, may be directly used as the coating liquid in the present invention. However, it is preferable that the organic phase in the solution is separated and then replaced with another solvent such as methyl-isobutyl-ketone and propylene-glycol-monopropyl-ether, by means of a rotary evaporator or the like, whereby the alcohol (including a by-product of the hydrolysis reaction), water and catalyst dissolved therein can be completely removed from the original solution, and the solid content of the new solution can be easily adjusted as described above.
The method of the present invention for forming a silica-containing film with a low-dielectric constant of 3 or less steadily on a semiconductor substrate comprises steps of:
(a) applying a coating liquid for forming a silica-containing film with a low-dielectric constant onto a semiconductor substrate,
(b) heating the thus coated film at 50 to 350xc2x0 C., and then
(c) curing the thus treated film at 350 to 450xc2x0 C. in an inert-gas atmosphere containing 500 to 15,000 ppm by volume of oxygen.
Further details of the above steps (a) to (c) are as described below.
(a) Coating Step
The coating liquid as described above can be applied onto the semiconductor substrate by the use of any customary techniques, such as spraying, spin coating, dip coating, roll coating or transfer printing, for forming a film coated thereon (i.e., a coated film).
(b) Thermal Treatment Step
The coated film formed on the semiconductor substrate by the above step (a) is then heated at a temperature of 50 to 350xc2x0 C., preferably 150 to 350xc2x0 C. The temperature for the thermal treatment is adequately selected from the level sufficiently high but lower than 350xc2x0 C. to vaporize the organic solvent contained in the coated film. Heating the coated film at a temperature of not higher than 350xc2x0 C. is undesirable, because it will cause rapid evaporation of the organic solvent contained in the film, and also rapid decomposition or evaporation of the readily decomposable resin. Time for the thermal treatment is 5 minutes or shorter, preferably 1 to 3 minutes, although varying depending on the film thickness and other properties.
The thermal treatment step (b) may be conducted under a uniform condition, e.g., (i) at 150xc2x0 C. for 3 minutes or (ii) at 250xc2x0 C. for 3 minutes, or may be conducted with different temperatures increased stepwise, e.g., (iii) at 150xc2x0 C. for 1 minute, 250xc2x0 C. for 1 minute and 350xc2x0 C. for 1 minute (i.e., step baking method).
The step baking method as described above (iii) causes the organic solvent contained in the coated film to vaporize slowly, thereby making the film thickness more uniform. It has another advantage of slowly decomposing or evaporating part of the readily decomposable resin, when it is contained in the coated film. However, it tends to cause the coating film formed on the semiconductor substrate to have a slightly higher dielectric constant than that obtained from the above method (i) which is effected at a relatively low temperature.
The thermal treatment step may be conducted in an inert-gas atmosphere such as nitrogen gas, as used in the conventional manner. However, the thermal treatment step (b) in the present invention is preferably conducted in an air atmosphere, especially by using of a dry air. Because the film is treated at a relatively low temperature for such a short time as described above, the metal wiring on the semiconductor substrate will not be damaged, for instance, by oxidation of the metal component, even if it is exposed to an air atmosphere containing oxygen at a relatively high proportion. Thus, the thermal treatment in the present invention can save the consumption of expensive nitrogen gas. And then the thermal treatment enables a trace quantity of oxygen to be easily incorporated into the coated film, which results in helping formation of SiO2 in the subsequent curing treatment step (c) as described above and in easily forming a low-dielectric, silica-containing film having a low moisture adsorptivity and high film strength.
The thermal treatment leads to the vaporization of the organic solvent contained in the coated film; the moderate curing of the solid components contained in the film by their polymerization and polycondensation being in progress; the lowering of the melt viscosity of the resultant polymer components; and the improvement of its re-flow property. As a result, a coating film with a highly enhanced planarity can be obtained.
Although it varies depending on the substance to be coated and its target, the thickness of the silica-containing film with a low-dielectric constant formed in the above manner generally ranges from about 1,000 to 2,500 xc3x85 when forming on a silicon wafer of a semiconductor device and generally ranges from 1,000 to 10,000 xc3x85 when forming between wiring layers of a multi-level interconnect.
(c) Curing Treatment Step
The thermally treated film as described above is then subjected to the curing treatment step (c) to be carried out at the temperature of 350 to 450xc2x0 C., preferably 375 to 425xc2x0 C. in an inert-gas atmosphere containing 500 to 15,000 ppm, preferably 1,000 to 10,000 ppm by volume of oxygen.
As the inert gas, nitrogen gas is preferably used. In the present invention, the inert gas is used with addition of oxygen gas or air (dry air) in an adequate quantity. The oxygen content of the inert gas is carefully adjusted in the range of 500 to 15,000 ppm by volume.
If the oxygen content is not more than 500 ppm by volume, the cured coating film may not have sufficient mechanical strength, which results in a poor practicality of the semiconductor substrate having such a film. It is likely that this is due to insufficient formation of the xe2x80x94Oxe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94 networks in the cured film treated under a condition of such an atmosphere.
On the other hand, if the oxygen content is not less than 15,000 ppm by volume, the metal wiring arranged on the semiconductor substrate may be easily damaged by oxidation of the metal component in the process of the curing treatment step (c), which results in decreasing the yield of final products of the semiconductor substrate.
The curing temperature varies depending on the properties of the silicon compound contained in the coating liquid as described above, but is selected from a range of the temperatures from 350 to 450xc2x0 C. For the coating liquid containing the above readily decomposable resin, the curing step (c) is preferably conducted at a higher temperature than that of which the resin is readily decomposed or vaporized. By such a treatment, a low-density silica-containing film with a low-dielectric constant and essentially containing no pores or voids of more than 10 nm (100 xc3x85) in diameter can be obtained.
Time for the curing treatment is desirably 10 to 60 minutes, although depending on, for example, type of the coating liquid and thickness of the coating film.
If the curing temperature is not higher than 350xc2x0 C., the coating film may not have a sufficient mechanical strength because of insufficient formation of the xe2x80x94Oxe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94 networks in the cured film. On the other hand, if the curing temperature is not lower than 450xc2x0 C., the metal wiring made of aluminum or copper may be oxidized or melted when the coating film is cured, thereby damaging the metal wiring arranged on the semiconductor substrate. Therefore, the curing temperature is to be in the range of 350 to 450xc2x0 C.
In the ordinary methods, the curing treatment step has been conducted normally in the atmosphere of an inert gas essentially consisting of nitrogen gas, or in the atmosphere of an inert gas containing a relatively high proportion of oxygen, e.g., 5% by volume. However, the inventors of the present invention have found that the curing treatment done under the above conditions stably gives, in high yield, the semiconductor substrate with a silica-containing film having the above characteristics.
This curing treatment step (c) may be conducted in a known vertical or horizontal furnace, which can simultaneously treat a number of the semiconductor substrates, because the inert gas (e.g., nitrogen) with a given amount of oxygen, which was adjusted in advance by addition of oxygen gas or air (dry air) thereto, is fed continuously into the furnace. However, it is essential in the present invention to control the oxygen content of the inert gas in the above range, and the coating film is preferably cured on a hot plate of a leaf-type curing unit with an upper lid. In addition, the unit for the curing treatment is preferably equipped with cooling means (e.g., cooling plate) to cool the cured film before it is taken out of the unit, in order to prevent the hot film from being exposed to the air atmosphere of higher oxygen content.
The semiconductor substrate having a silica-containing film with a low-dielectric constant in the present invention is one of the parts for a semiconductor device, at which the coating film is formed by the above specific method on a silicon wafer, a part between wiring layers of a multi-level interconnect, an element surface and/or a part of PN junction.
The coating film formed on the semiconductor substrate by the above method of the present invention has a low-dielectric constant of 3 or less, and is not only excellent in the adhesion to the substrate, resistance to chemicals (e.g., alkali), anti-cracking, and process suitability (e.g., in the resistance to oxygen plasma and etching processability), but also in the low moisture adsorptivity and high film strength.
Accordingly, in the present invention the semiconductor substrate having the silica-containing film with the excellent characteristics as described above is steadily obtained without causing any damage to the metal wiring arranged on the substrate.